. "Protein involved in the synthesis of tetrahydrobiopterin, the reduced form of dihydrobiopterin, a reduced pteridine derivative related to folic acid. It is a coenzyme for the enzyme phenylalanine-4-hydroxylase which is involved in phenylalanine degradation." . . "Tetrahydrobiopterin biosynthesis" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .